The connecting link between glycolysis and Kreb's cycle in aerobic respiration is?

KPSC Staff Nurse - 2016

Biochemistry & Nutrition

Easy

Appeared in: KPSC Staff Nurse - 2016

Explanation

Acetyl-CoA is the molecule that serves as the bridge between glycolysis and the Krebs cycle.
Glycolysis ends with the production of pyruvate in the cytoplasm.
Pyruvate is then transported into the mitochondria and converted into Acetyl-CoA through a process called the link reaction (oxidative decarboxylation).
This Acetyl-CoA molecule then enters the Krebs cycle by donating its two-carbon acetyl group to oxaloacetate.

Option A: RUBP (Ribulose-1,5-bisphosphate) is a molecule central to the Calvin cycle in photosynthesis, where it combines with CO₂. It has no role in animal cellular respiration.
Option C: Citric acid is the first product formed within the Krebs cycle when Acetyl-CoA combines with oxaloacetate. It is a component of the cycle itself, not the link leading into it.
Option D: Oxaloacetic acid is the molecule that accepts the acetyl group from Acetyl-CoA to initiate the Krebs cycle. It is the starting and ending point of the cycle, but not the link from glycolysis.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Biochemical pathway linking glycolysis and the Krebs cycle as background academic context rather than a clinical decision trigger.
Understanding the link reaction is crucial for diagnosing and managing metabolic disorders. A defect in the Pyruvate Dehydrogenase (PDH) complex, which catalyzes this step, can block aerobic respiration.
Thiamine (Vitamin B1) deficiency, common in individuals with chronic alcoholism, impairs the PDH complex, leading to a buildup of pyruvate and its conversion to lactate, causing potentially fatal lactic acidosis.
Nurses monitor for signs of lactic acidosis (e.g., tachypnea, nausea, abdominal pain) in at-risk patients and may be involved in administering thiamine supplementation.

How to Approach the Question

First, identify the core of the question: it asks for the 'connecting link' between two specific stages of aerobic respiration - glycolysis and the Krebs cycle.
Recall the overall sequence of aerobic respiration: Glycolysis -> Link Reaction -> Krebs Cycle -> Electron Transport Chain.
Remember that glycolysis produces pyruvate. The Krebs cycle starts with the entry of a 2-carbon molecule.
Focus on the 'Link Reaction'. This is the step that converts the 3-carbon pyruvate into the 2-carbon molecule that enters the Krebs cycle.
Identify the molecule produced in this link reaction. Pyruvate is decarboxylated (loses a CO₂) and combined with Coenzyme A to form Acetyl-CoA.
Evaluate the options: Acetyl-CoA directly fits this description as the product of the link reaction and the substrate for the Krebs cycle, making it the correct connecting link.

Concept Tested & Keywords

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E6 Textbook of Biochemistry for medical StudentsDM Vasudevan Part 1 (pp 26-370 of 370) pp. 119-121, 118-120

E6 textbook of Applied Biochemistry and Nutrition & DieteticsHarbans Lal (pp 26-464 of 479) p. 30-32